One goal for Python 3000 should be to simplify the language by
removing unnecessary or duplicated features. There are currently
several ways to indicate that a logical line is continued on the
following physical line.

The other continuation methods are easily explained as a logical
consequence of the semantics they provide;
\
is simply an escape
character that needs to be memorized.

A terminal
\
indicates that the logical line is continued on the
following physical line (after whitespace). There are no particular
semantics associated with this. This form is never required, although
it may look better (particularly for people with a C language
background) in some cases:

>>> assert val>4, \
"val is too small"

Also note that the
\
must be the final character in the line. If
your editor navigation can add whitespace to the end of a line, that
invisible change will alter the semantics of the program.
Fortunately, the typical result is only a syntax error, rather than a
runtime bug:

>>> assert val>4, \
"val is too small"
SyntaxError: unexpected character after line continuation character

This PEP proposes to eliminate this redundant and potentially
confusing alternative.

A terminal
\
within a single-quoted string, at the end of the
line. This is arguably a special case of the terminal
\
, but it
is a special case that may be worth keeping.

>>> "abd\
def"
'abd def'

Pro: Many of the objections to removing
\
termination were
really just objections to removing it within literal strings;
several people clarified that they want to keep this literal-string
usage, but don't mind losing the general case.

Pro: The use of
\
for an escape character within strings is well
known.

Contra: But note that this particular usage is odd, because the
escaped character (the newline) is invisible, and the special
treatment is to delete the character. That said, the
\
of
\(newline)
is still an escape which changes the meaning of the
following character.

Several people have suggested alternative ways of marking the line
end. Most of these were rejected for not actually simplifying things.

The one exception was to let any unfinished expression signify a line
continuation, possibly in conjunction with increased indentation.

This is attractive because it is a generalization of the rule for
parentheses.

The initial objections to this were:

The amount of whitespace may be contentious; expression continuation
should not be confused with opening a new suite.

The "expression continuation" markers are not as clearly marked in
Python as the grouping punctuation "(), [], {}" marks are:

# Plus needs another operand, so the line continues
"abc" +
"def"
# String ends an expression, so the line does not
# not continue. The next line is a syntax error because
# unary plus does not apply to strings.
"abc"
+ "def"

Guido objected for technical reasons.
[1]
The most obvious
implementation would require allowing INDENT or DEDENT tokens
anywhere, or at least in a widely expanded (and ill-defined) set of
locations. While this is of concern only for the internal parsing
mechanism (rather than for users), it would be a major new source of
complexity.

Andrew Koenig then pointed out
[2]
a better implementation
strategy, and said that it had worked quite well in other
languages.
[3]
The improved suggestion boiled down to:

The whitespace that follows an (operator or) open bracket or
parenthesis can include newline characters.

It would be implemented at a very low lexical level -- even before
the decision is made to turn a newline followed by spaces into an
INDENT or DEDENT token.

There is still some concern that it could mask bugs, as in this
example
[4]
:

# Used to be y+1, the 1 got dropped. Syntax Error (today)
# would become nonsense.
x = y+
f(x)

Requiring that the continuation be indented more than the initial line
would add both safety and complexity.